Anxiety disorders are common among youth, with prevalence estimates around 13%. Though these disorders can occur alone, they are often comorbid and are associated with a variety of adverse outcomes. Anxiety disorders in childhood put individuals at greater risk for educational underachievement, substance abuse, anxiety, and depression later in life. These disorders rarely remit on their own, making understanding their treatment a priority of great public health importance. Cognitive behavioral therapy (CBT) is a well- established empirically supported treatment for childhood anxiety, typically achieving response rates of 60% in clinic-based trials. However, there remains a large portion of youth who do not respond to CBT interventions and little is known about the mechanisms of change in CBT for youth anxiety. Thus, there is a need for research examining what factors may differentiate treatment responders from nonresponders, as well as the mechanisms through which CBT for childhood anxiety achieves its effects. Existing research aimed at understanding these factors has tended to take a nomothetic approach to analyses, aggregating information from large samples to understand what predicts outcome for the average youth. However, it is unlikely that many youth present to clinics similarly to the average youth, which may explain why the results of current studies have been inconclusive. The present study uses a dynamical systems framework to examine mechanisms of change in CBT for childhood anxiety both idiographically (individual level) and nomothetically (group level). We propose to collect ecological momentary assessment (EMA) data via a smartphone application (app) at pretreatment (Time 1), midtreatment (Time 2), and posttreatment (Time 3) from 80 youth aged 11 - 17 receiving treatment at the Child and Adolescent Anxiety Disorders Clinic. At Time 1 and Time 3, youth and their families will also undergo a diagnostic interview to assess for psychopathology, as well as complete a variety of independent-evaluator administered, parent-, and self-report measures of anxiety severity, coping, emotion regulation, and depressive symptoms. A dynamical systems framework will guide the analytical approach and will allow the simultaneous examination of the relationship between and across variables to model potential mechanisms of change. Results from the present study will inform the mechanisms of change of CBT for childhood anxiety, as well as inform the personalization of treatment, ultimately reducing costs of mental health care.